Touch sensitive devices can be implemented to allow a user to interface with electronic systems and displays conveniently, for example, by providing a display input that is typically prompted by a visual in the display for user-friendly interaction and engagement. In some instances, the display input complements other input tools such as mechanical buttons, keypads and keyboards. In other instances, the display input acts as an independent tool for reducing or eliminating the need for mechanical buttons, keypads, keyboards and pointing devices. For example, a user can carry out a complicated sequence of instructions by simply touching an on-display touch screen at a location identified by an icon or by touching a displayed icon in conjunction with another user input.
There are several types of technologies for implementing a touch sensitive device including, for example, resistive, infrared, capacitive, surface acoustic wave, electromagnetic, near field imaging, etc., and combinations of these technologies. Touch sensitive devices that use capacitive touch sensing devices have been found to work well in a number of applications. In many touch sensitive devices, the input is sensed when a conductive object in the sensor is capacitively coupled to a conductive touch implement such as a user's finger. Generally, whenever two electrically conductive members come into proximity with one another without actually touching, a capacitance is formed therebetween. In the case of a capacitive touch sensitive device, as an object such as a finger approaches the touch sensing surface, a tiny capacitance forms between the object and the sensing points in close proximity to the object. By detecting changes in capacitance at each of the sensing points and noting the position of the sensing points, the sensing circuit can recognize multiple objects and determine the characteristics of the object as it is moved across the touch surface.
Different techniques have been used to measure touch based on such capacitive changes. One technique measures change in capacitance-to-ground, whereby the status of an electrode is understood based on the capacitive condition of a signal that is applied to the electrode before a touch would alter the signal. A touch in proximity to the electrode causes signal current to flow from the electrode, through an object such as a finger or touch stylus, to electrical ground. By detecting the change in capacitance at the electrode and also at various other points on the touch screen, the sensing circuit can note the position of the points and thereby recognize the location on the screen where the touch occurred. Also, depending on the complexity of the sensing circuit and related processing, various characteristics of the touch can be assessed for other purposes such as determining whether the touch is one of multiple touches, and whether the touch is moving and/or satisfies expected characteristics for certain types of user inputs.
Another known technique monitors touch-related capacitive changes by applying a signal to a signal-drive electrode, which is capacitively coupled to a signal-receive electrode by an electric field. As these terms connote, with the signal-receive electrode returning an expected signal from the signal-drive electrode, an expected signal (capacitive charge) coupling between the two electrodes can be used to indicate the touch-related status of a location associated with the two electrodes. Upon or in response to an actual or perceived touch at/near the location, the status of signal coupling changes, and this change is reflected by a reduction in the capacitive coupling.
For these and other related capacitive-touch sensing techniques, various methodologies have been used to measure the mutual capacitance between electrodes. Depending on the applications, these methodologies might specify different types and speeds of signals through which the signal-drive electrode would provide the expected signals to the signal-drive electrode, from which a change in capacitive charge is sensed. With the growing trend in higher-speed electronics, many such applications are requiring that relatively higher-frequency signals be used for driving the signal-drive electrodes. Unfortunately, RF (radio-frequency) interference can ensue from both the higher-speed electronics and such signals generated therefrom. This RF interference can degrade and, in some applications, can undermine the effectiveness of the sensing circuits and related processing for the associated touch display. Adverse effects can include speed of detection, accuracy and power consumption.
The above issues are examples of those that have presented challenges to the effective designs of touch-sensitive displays and related methods for locating and assessing the touches.